In the traditional powder-metallurgy art, it has long been a practice to heat a precompacted or uncompacted mass of particles to obtain a sintered object therefrom. According to a known process, however, sintered end products of desired quality may not be obtained or certain materials may not lend themselves to satisfactory sintering. In another known sintering technique which is commonly called "hot pressing", a mass of particulate material is placed in a mold such as of graphite and therein compressed between a pair of punches while simultaneously being heated with heating current passing directly through the mass, through the mold or through an induction heating coil arranged to surround the mass and mold. The hot pressing is sometimes carried out with compaction force applied uniformly to the mass peripherally toward central point or axis thereof.
In my copending U.S. Patent Application Ser. No. 27,662 filed Apr. 6, 1979 now U.S. Pat. No. 4,273,581 it has been pointed out that in any of the conventional techniques, a problem has now been recognized that directional variation develops in the quality of a sintered product due to the fact that sintering crystals in the mass are forced to grow with their easy-to-slip surfaces aligned in the direction in which the pressure is applied to the mass. Thus, insufficiencies in the density and lack of uniformity in the quality of sintered products result, this being particularly noticeable where the sintered particles have a tendency to generate a large quantity of gaseous decompositions.
In the aforementioned copending application, there is described an improved method of sintering a mass of particles in which a pressure is applied externally to the mass along a plurality of axes to multi-directionally compress the mass while the mass is being heated by the direct passage of a high-amperage resistive heating current or the inductive application of a heating current through the mass.